And thanks to indefatigable email newsletter author James Counts, CPA, in California, I learned today that IRS is out with a draft of a September 2021 version of 1040-X, and its instructions include this morsel:

Continuous-use form and instructions. Form 1040-X and its
instructions have been converted from an annual revision to
continuous use beginning in tax year 2021. Both the form and
instructions will be updated as required. For the most recent
version, go to IRS.gov/Form1040X. Section discussions and
charts that were updated annually have been removed, or
replaced with references to relevant forms, schedules,
instructions, and publications. See the forms, schedules,
instructions, and publications for the year of the tax return you
are amending for guidance on specific topics.